Radiation containing seeds and method for high visibility magnetic imaging

ABSTRACT

A radioactive seed and method for making a radioactive seed with selective magnetic imaging characteristics are provided. The seed includes a housing which may include a metal shell for at least partially enclosing a radioactive material. The shell encloses a rod having a nickel layer with a phosphorous content, wherein the phosphorous content includes a level of phosphorous sufficient, when the seed is implanted in tissue, to provide a magnetic resonance image of the seed while substantially eliminating gross artifacts in the magnetic resonance image.

PRIORITY

This application is a continuation of U.S. patent application Ser. No.13/524,763, filed Jun. 15, 2012, now U.S. Pat. No. 8,834,340, which is acontinuation of U.S. patent application Ser. No. 12/371,004, filed Feb.13, 2009, which claims the benefit of priority to U.S. ProvisionalApplication No. 61/030,735, filed Feb. 22, 2008, each of which isincorporated by reference in its entirety into this application.

TECHNICAL FIELD

The invention relates generally to radioactive seeds and moreparticularly to radioactive seeds formulated for improved magneticresonance imaging.

BACKGROUND

Bodily cancers are commonly treated using radiation therapy. Radiationtherapy employs high energy radiation to kill cancer cells. One type ofradiation therapy is brachytherapy, in which a source of radiation is indirect contact with an afflicted tissue. A common brachytherapytreatment, transperineal seed implantation, involves placing radioactiveseeds in the prostate gland to kill prostate gland cancer cells. Aphysician employs tools, for example, ultrasound, computed axialtomography (“CAT” or “CT”) scans, magnetic resonance imaging (“MRI”)scans and X-ray images in concert with dose-planning computer softwareprograms to evaluate the medical condition of a patient. The physicianconstructs an optimal treatment plan to evenly distribute radiationthroughout the afflicted tissue. Radioactive seeds of discreteradioactive strengths are inserted into the afflicted tissue throughmultiple implantation needles at positions corresponding to thetreatment plan.

During prostate brachytherapy, the position of the radioactive seeds inrelation to the prostate gland and to adjacent structures in the bodymust be known to a relatively high degree of precision to accuratelydetermine if the dose of radiation delivered from the seeds is adequateto eradicate the prostate cancer. Currently, seeds are guided into theprostate using transrectal ultrasound guidance which is good for imagingthe soft tissue of the prostate and the surrounding structures.Occasionally fluoroscopy is used in conjunction with the ultrasound toprovide improved images of the seeds. Following the implant, the seedposition is typically determined using CT imaging. Using the determinedseed positions, the dosimetry of the implant is calculated based on theobtained CT images. The CT images have been shown however to be deficitin imaging soft tissue structures, therefore, performing ultrasoundimaging or MRI imaging and fusing the ultrasound or MRI images with theCT images is often performed.

Many of the current modalities have shortcomings that need to beovercome before an accurate determination of implant location can bemade. For example, ultrasound can determine the structure of the glandand surrounding anatomy to a high degree of accuracy. However,ultrasound is not very accurate at imaging the seeds. Various attemptshave been made to design the seeds to be more echogenic (i.e., moreaccurately imaged by ultrasound). The brachytherapy seed sold asONCOSEED™ (e.g., Iodine-125 brachytherapy seed) by Oncura uses ribs onan outer titanium hull of the seed to increase its ultrasonicreflection. Other attempts have been made to incorporate gas bubblesinto the walls of the seeds or into stranding material holding the seedstogether to enhance the ultrasound reflection. These attempts have beenminimally successful primarily due to the small target size of the seed,the background noise caused by the other seeds in the implant, theechogenic needle tracks created in the gland and the like.

Other methods for locating and imaging the seeds also have shortcomings.Fluoroscopy is very good at identifying the seeds due to the heavy metalX-ray markers in the seeds. Unfortunately, fluoroscopy is poor atimaging the soft tissue in and around the prostate gland. CT imaging isacceptable at imaging both the seeds and the soft tissues, but it is farfrom ideal. CT imaging is typically not refined enough to image criticalstructures around the gland and therefore, generally used for grossposition analysis only. CT imaging is most often used to do post implantdosimetry. Studies have shown that there is a great deal ofoperator-to-operator variability, however, in contouring the prostateshape and size for a given image. This variability leads to subsequentvariations in the determination of the adequacy of the dose delivered.

MRI imaging is the preferred method for imaging the prostate anatomy.Identification of critical structures located around the prostate glandcan be performed using MRI imaging. MRI imaging is however rather poorat imaging the seed positions following an implant. Uncontrolledartifacts for the seed can also distort the gland image and makeaccurate dosimetry assessment difficult. In the presence of sufficientartifacts, the seeds may be invisible to MRI imaging. Additionally, thedegree of artifact, or visibility, for a given seed type varies fromvendor to vendor and, possibly, even from lot to lot from the samevendor.

Medical professionals therefore are required to compensate for deficitsin one or more of the available imaging technologies by employingmultiple imaging techniques and fusing the resulting images to determinean accurate location of the implant.

Thus, a need exists for a radioactive seed conducive to providing highfidelity during a magnetic resonance imaging procedure while reducingartifacts caused by the seed in the image.

SUMMARY

The invention in one implementation encompasses a radioactive seedmanufactured to have desired magnetic characteristics. The seed maycomprise a metal shell for at least partially enclosing a radioactivematerial. A rod traverses the shell and contains a nickel layer. Thenickel layer has a phosphorus content which is sufficient, when the seedis implanted in tissue, to provide a magnetic resonance image of theseed while substantially eliminating gross artifacts in the image.

Another implementation of the invention encompasses a method for makinga radioactive seed to have desired magnetic characteristics. The methodmay comprise providing a rod for insertion in a shell of a radioactiveseed. The method may comprise the steps of: coating the rod with anickel layer; providing incorporation of phosphorus into the nickellayer; and adjusting the concentration of the phosphorous such that whenthe seed is placed in tissue and a magnetic resonance image is takenthereof, a level of phosphorous in the nickel layer is sufficient tosubstantially eliminate gross artifacts in the magnetic resonance imagewhile providing visibility of the seed on the magnetic resonance image.

DESCRIPTION OF THE DRAWINGS

Features of exemplary implementations of the invention will becomeapparent from the description, the claims, and the accompanying drawingin which:

FIG. 1 is a representation of an exemplary implementation of anapparatus that comprises a radioactive seed.

DETAILED DESCRIPTION

While this invention is susceptible of embodiment in many differentforms, there is shown in the drawing and will herein be described indetail preferred embodiments of the invention with the understandingthat the present disclosure is to be considered as an exemplification ofthe principles of the invention and is not intended to limit the broadaspect of the invention to the embodiments illustrated.

Turning to FIG. 1, an apparatus 100 comprising a radioactive seed 110for inserting into a prostate organ for treating, for example, prostatecancer is shown. The radioactive seed 110 in accordance with anembodiment of the invention is manufactured to have desired magneticcharacteristics. The seed 110 comprises a metal shell 120 that encloses,at least partially, a radioactive material 130. The radioactive material130 is used to irradiate cancers in a well known manner. The metal shell120 may be comprised of any appropriate material, such as titanium. Theradioactive seed 110 further comprises a rod 140 traversing the shell120. The metal shell 120 and the rod 140 may comprise a housing for atleast partially enclosing the radioactive material. The rod 140 may bemade of a number of layers. For example, the rod 140 may comprise a goldcore 150 surrounded by a layer of aluminum 160. The layer of aluminum160 may be surrounded by consecutive layers of nickel 170, copper 180and the radioactive material 130, such as radioactive iodine I¹²⁵. C.R.Bard manufactures a similar radioactive seed as the STM 1251 ¹²⁵I seed.An air gap 190 may be filled with an inert gas, such as argon.

The rod 140 traversing the shell 120 preferably has a nickel layer witha phosphorous content wherein the phosphorous content comprises a levelof phosphorous sufficient, when the seed 110 is implanted in tissue, toprovide a magnetic resonance image of the seed 110 while substantiallyeliminating gross artifacts in the magnetic resonance image. Forexample, the layers of nickel 170 and copper 180 are commonly andpreferably plated to the layer of aluminum 160. The electroplatingmethod used to deposit the layer of nickel 170 influences the magneticcharacteristics of the rod 140 and therefore affects the visibility ofthe rod 140 under magnetic resonance imaging (MRI). If the rod 140 isplated using a phosphorous-free bath, the layer of nickel 170 exhibitsmagnetic characteristics which may cause a relatively large, undesirableartifact on a MRI image of the seed 110. Having such artifacts makes anaccurate dosimetry assessment difficult. Undesirable artifacts mayfurther complicate assessments in that they may also distort the glandimage.

If the rod 140 is plated using a phosphorous bath, the layer of nickel170 becomes increasingly non-magnetic as the phosphorous contentincreases. If the phosphorous content of the layer of nickel 170 becomesgreat enough, the layer of nickel 170 becomes essentially non-magnetic.The relationship between the level of phosphorous and the magneticcharacteristics of nickel are discussed in the Standard Specification ofAutocatalytic (Electroless) Nickel-Phosphorous Coatings on Metal,published by ASTM International as designation B 733-04, publishedAugust 2004, the disclosure of which is hereby incorporated in itsentirety by reference. In accordance with an aspect of this invention,the phosphorous content of the layer of nickel 170, and more generallythe rod 140, is adjusted to provide enough magnetic character to theseed 110 to allow for imaging and localization, but not enough magneticcharacter to cause gross artifacts which would interfere with detailedimaging of the prostate gland. Therefore, the electroplating process isused to adjust the phosphorous content of the rod 140 to provide desiredmagnetic resonance image characteristics.

A method for making the radioactive seed 110 to have desired magneticcharacteristics is also provided. The rod 140 is inserted in the shell120 of the radioactive seed 110. In general, the rod 140 may be exposedto, or submersed in, a nickel-containing bath. The phosphorous presentin the bath is incorporated into the nickel layer as it is formed on therod 120 as it is submersed. As previously explained, the level ofphosphorous contained in the nickel layer 170 of rod 140 effects themagnetic characteristics of the rod 140. Therefore, the phosphoruscontent is adjusted by standard means such that when the seed 110 isplaced in tissue and a magnetic resonance image is taken thereof, thelevel of phosphorous contained in nickel layer 170 of rod 140 issufficient to substantially eliminate gross artifacts in the magneticresonance image while providing visibility of the seed 110 on themagnetic resonance image. Adjustment of the phosphorous level in thenickel layer 170 of rod 140 may be made by increasing or decreasing thepercentage of phosphorous in the bath or the time the rod 140 issubmersed. The level of phosphorous may be adjusted so that visibilityof the seed 110 in tissue on a magnetic resonance image is maximized.Determining the proper level of phosphorous may be advantageouslydetermined by any appropriate method. For example, seeds 110 may bemanufactured using rods 140 having varying degrees of phosphoruscontent. The seeds 110 may be inserted into tissue and magneticresonance images of the seeds 110 may be generated, preferably by MRI.Based on one or more characteristics of the magnetic resonance images,such as the size of artifacts, visibility of the seed 110 and the like,the optimal level of phosphorous in the rods 140 may be determined andoptimized seeds produced via the methods disclosed above.

The steps or operations described herein are just exemplary. There maybe many variations to these steps or operations without departing fromthe spirit of the invention. For instance, the steps may be performed ina differing order, or steps may be added, deleted, or modified. The MRIsignature of a seed may be varied according to the amount of containedactivity (e.g. very radioactive seeds may be made to have largerartifacts than less radioactive seeds, so different activity seeds maybe identified during a MRI scan.) Seeds of different MRI visibility maybe used in different areas of the prostate, such as having very visibleseeds in the interior of the gland and less visible seeds in theperiphery of the prostate to minimize interference from the peripheralseeds when imaging the interior of the gland. If the seeds were easilyseen on a MRI scan, it may be possible to do the implant withoutfluoroscopy or CT. In such a case, the seeds would not need a heavymetal gold marker, which would reduce the amount of activity needed inthe seeds and therefore reduce cost. By varying the magnetic characterof the seeds, it may be possible to move the seeds through anapplication of an external magnetic field which would permitrepositioning after implantation.

In accordance with one embodiment, a method for making a radioactiveseed may comprise providing a housing which substantially encloses theradioactive material. The magnetic character of the housing iscontrolled to obtain a desired magnetic character. A number of methodsmay be employed to improve, or modify, the magnetic characteristics ofthe radioactive seed 110. The magnetic character of the radioactive seedmay be controlled by varying the metallic composition of the housing, orseed. For example, an amount of iron incorporated into the housing maybe varied to provide a desired magnetic character. Types of materialsincorporated into the housing may be selected to provide the desiredmagnetic character to the radioactive seed. For example, one or moretypes of stainless steel may be selected for incorporation into thehousing to provide the desired magnetic character. The magneticcharacter of the radioactive seed may be controlled via other processes.For example, an iron-containing seed may be subjected to a magneticfield at a predetermined strength for a predetermined time period inorder to obtain the desired magnetic character.

By maximizing the visibility of the seed 110 on a magnetic resonanceimage while minimizing gross artifacts on the image, the MRI may be ableto provide both anatomic details and accurate seed position. Accuratemapping of both structure and seed position would possibly allow fordefinitive post-implant quality control, as well as provide a potentialfor real-time imaging of the quality of the implant using MRI to theensure the radiation dose delivered to critical structures around theprostate is minimized. These critical structures, including the penilebulb and neurovascular bundles, are difficult to locate usingconventional ultrasound imaging but may be easily viewed using magneticresonance imaging. Accurate dosimetry can be calculated without the needfor (and the inherent error in) fusing two images together. There wouldtherefore likely be no need for a second scan or fusion software. Suchaccurate imaging further saves time for the patient and the physicianand would reduce patient radiation exposure if a post-implant CT is notneeded or if fluoroscopy during the procedure is reduced.

Although exemplary implementations of the invention have been depictedand described in detail herein, it will be apparent to those skilled inthe relevant art that various modifications, additions, substitutions,and the like can be made without departing from the spirit of theinvention and these are therefore considered to be within the scope ofthe invention as defined in the following claims.

What is claimed is:
 1. A method for controlling a size of an artifact animplantable brachytherapy seed produces on a magnetic resonance imagewhen implanted, wherein the implantable brachytherapy seed comprises ametal rod, a radioactive material, and a shell, the method comprising:plating the metal rod in a nickel bath comprising phosphorus; adjustinga level of phosphorus in the nickel bath to substantially eliminategross artifacts in a magnetic resonance image while providing visibilityof the brachytherapy seed on the magnetic resonance image; and enclosingthe metal rod and the radioactive material in the shell.
 2. The methodaccording to claim 1, wherein the brachytherapy seed has a magneticcharacter that is strong enough to provide a magnetic resonancesignature that helps define a seed position in the magnetic resonanceimage, but not so strong as to distort an image of a body region.
 3. Themethod according to claim 1, wherein the metal rod comprises a gold coreand layers of aluminum, nickel, and copper.
 4. The method according toclaim 3, wherein the nickel layer is disposed between the aluminum layerand the copper layer.
 5. The method according to claim 3, wherein theradioactive material is disposed on the copper layer.
 6. The methodaccording to claim 1, wherein an air gap is formed between theradioactive material and the shell, further comprising filling the airgap with an inert gas.
 7. The method according to claim 6, wherein theinert gas is argon, and wherein enclosing the metal rod and theradioactive material in the shell includes filling the air gap withargon.
 8. The method according to claim 1, wherein the radioactivematerial is radioactive iodine.